The present invention relates to insulated gate semiconductor devices and more particularly to those insulated gate devices which are susceptible to achieving a non-preferred latched state of operation. Insulated gate devices which are susceptible to latching include insulated gate transistors, insulated gate thyristors, and other MOS controlled devices which have one or more inherent bipolar transistors included therein. Under latched operating circumstances, the base emitter junction of the inherent transistor can become forward biased causing the device to continue to conduct current even though the gate drive has been turned off. Thus, insulated gate control of the device can be lost and it is then generally necessary to remove the forward bias potential and/or commutate the device to stop current flow and to turn the device off.
Referring now to FIG. 1, a conventional insulated gate transistor comprising four layers of alternate conductivity material is shown. A cathode electrode is disposed in ohmic contact with the base and souce regions. An insulated gate is disposed over the base and conductively couples opposite type conductivity carriers from the cathode through a base region gate induced channel into the drift region of the device. At the same time, one type conductivity carriers flow through the base and drift regions between the anode and the cathode electrode. One type conductivity carriers, however, flowing along the PN junction between the base and emitter regions can establish a voltage drop V along this junction. In the event the voltage drop along this junction exceeds approximately 0.7 volts in a silicon device, the junction can become forward biased causing the upper NPN transistor to conduct. Once activated, the upper transistor establishes a regenerative conduction relationship with the lower PNP transistor so that the device as a whole functions as a silicon controlled rectifier or thyristor which is regeneratively latched into a conductive state. Insulative gate control of this device is thus lost.